1. Field of the Invention
The present invention relates to an electrode catalyst for use in fuel cells which is improved in the catalytic activity of a noble metal such as platinum by using a carbonaceous material having a particular structure, and to a fuel cell in which such catalyst is used.
2. Description of the Prior Art
In solid polymer type fuel cells, the cell to be inserted in a cell module comprises a sheet-form solid polyelectrolyte membrane, an anode (fuel electrode) and a cathode (oxidizing agent electrode) oppositely disposed in a manner sandwiching the solid polyelectrolyte membrane, as well known in the art.
The fundamental constitution of the unit cell in conventional solid polymer type fuel cells is as shown in the exploded view given in FIG. 9. The unit cell proper (membrane/electrode assembly) is constructed by intimately joining, by means of a hot press, a cathode side catalyst layer 2, and an anode side catalyst layer 3 each comprising carbon black particles and particles of a noble metal (mostly platinum (Pt) or a platinum group metal (Ru, Rh, Pd, Os, Ir)) supported thereon respectively to both the main faces of a sheet-like solid polyelectrolyte membrane 1. Oppositely to the catalyst layer 2 and catalyst layer 3, respectively, there are disposed a cathode side gas diffusion layer 4 and an anode side gas diffusion layer 5 each structurally comprising a carbon paper, a woven carbon cloth or a like material coated with a mixture of carbon black and polytetrafluoroethylene (PTFE). Thereby constituted are a cathode 6 and an anode 7, respectively.
These gas diffusion layers 4 and 5 serve to feed and discharge an oxidizing agent gas (e.g. air) and a fuel gas, such as natural gas, city gas, methanol, LPG or butane, respectively, and at the same time function as electric current collectors for feeding electricity to the outside. The unit cell proper is sandwiched between a pair of separators 10 made of a conductive and gas-impermeable material and equipped with gas channels 8 facing to the unit cell proper and cooling water channels 9 on the opposite main faces. A unit cell 11 is thus constituted.
Employed as the solid polyelectrolyte membrane 1 are sulfonic acid group-containing polystyrenic cation exchange membranes used as cationic conductive membranes, and fluorine-containing ion exchange resin membranes, typically membranes made of a mixture of a fluorocarbonsulfonic acid and polyvinylidene fluoride, membranes produced by grafting trifluoroethylene onto a fluorocarbon matrix, and perfluorosulfonic acid resin membranes (e.g. Nafion™ membranes, products of DuPont). These solid polyelectrolyte membranes 1 contain proton exchange groups in each molecule and, when the water content arrives at a saturation level, the specific resistance at ordinary temperature becomes 20 Ωcm or below and they thus function as proton conductive electrolytes.
When reactant gases are fed to the above electrodes 6 and 7, respectively, three-phase interfaces involving a gaseous phase (reactant gas), a liquid phase (solid polyelectrolyte membrane) and a solid phase (catalyst supported on each electrode) are formed at the boundaries between the platinum group noble metal-supporting catalyst layers 2, 3 and the solid polyelectrolyte membrane 1, and allow electrochemical reactions to proceed, which lead to direct current electric power generation.
The electrochemical reactions include the following:    On the anode side: H2→2H++2e−    On the cathode side: 1/2O2+2H++2e−→H2OThe H+ ions formed on the anode (7) side migrate to the cathode (6) side through the solid polyelectrolyte membrane 1, while electrons (e−) migrate to the cathode (6) side through an external load.
On the other hand, on the cathode (6) side, oxygen contained in the oxidizing agent gas reacts with the H+ ions and electrons (e−) coming from the anode (7) side to form water. Thus, the solid polyelectrolyte type fuel cell generates direct electric current from hydrogen and oxygen while forming water.
In the above-described conventional solid polymer type fuel cell, such an expensive catalyst as platinum or a platinum-based alloy catalyst (e.g. Pt—Fe, Pt—Cr, Pt—Ru) is used in the electrodes in a relatively large amount, for example about 1 mg/cm2 per cell and, therefore, the electrode catalyst cost forms a large proportion of the cell module cost. Therefore, to reduce the usage of the noble metal catalyst is now one of the important tasks in putting fuel cells to practical use.
Various methods of achieving reductions in noble metal catalyst usage have been investigated to fulfill such task. One of them thus proposed comprises forming electrodes using a high density dispersion of noble metal catalyst particles with several nanometers (nm) in size on carbon black having a large specific surface area. However, this method still has problems in that there arises a problem of catalytic activity lowering due to catalyst sintering or elution and in that about 0.5 to 1 mg/cm2 of a noble metal catalyst is still required, hence the high cost problem is not solved as yet.
The use, as a platinum substitute catalyst, of an organometallic compound having a chelate structure or a metal oxide having a pyrochlore-type structure has also been investigated (cf. Japanese Kokai Publication Sho. 57-208073 or U.S. patent application Publication No. 2003-175580). In the actual circumstances, however, these substitutes are low in catalytic activity as compared with platinum catalysts.
The present inventors previously found that a phase having a graphite-like structure that has developed like an onion-like laminate around each metal particle (e.g. iron particle) having a size of the order of nanometers which structure is obtained by carbonizing a material capable forming hardly graphitizable carbon with an iron group metal compound, such as ferrocene, added thereto (hereinafter such phase is referred to as “carbon nanoonion phase”) and proposed that a carbonaceous material having such carbon nanoonion phase be used as an electrode catalyst for fuel cells (cf. Japanese Patent Application 2002-050381 or Japanese Kokai Publication 2003-249231).
However, those electrode catalysts for fuel cells in which the above-mentioned carbonaceous material is used have a problem in that their catalytic activity is low as compared with the conventional electrode catalysts comprising a noble metal catalyst supported on carbon black, hence the desired reduction in noble metal catalyst usage cannot be attained.
Accordingly, it is an object of the present invention to provide a higher activity electrode catalyst for fuel cells by improving the catalyst activity of an expensive noble metal catalyst, such as a platinum catalyst, using a less expensive material while reducing the usage of the noble metal catalyst.
Another object of the invention is to provide a fuel cell in which such high activity electrode catalyst is used.
A third object of the invention is to provide a membrane-electrode assembly resulting from lamination of one or two catalyst layers in which the above high activity electrode catalyst is used with a solid polyelectrolyte membrane.
A fourth object of the invention is to provide a gas diffusion electrode resulting from lamination of a catalyst layer in which the above high activity electrode catalyst is used with a gas diffusion layer.